PROJECT SUMMARY/ABSTRACT Organisms such as zebrafish and neonatal mice are capable of complete heart regeneration following partial amputation. Although adult humans and adult mice lack this cardiac regeneration response, there is great interest in understanding how heart regeneration can occur in lower organisms so that we can activate these processes in humans to better treat patients following myocardial infarction (MI). Cytokines and growth factors play a significant role in the initiation of tissue and organ regeneration in large part by directly stimulating proliferation of resident cells, or by recruitment and activation of wound healing inflammatory cells. The type II cytokine Interleukin 13 (IL13) activates pro-proliferative signaling molecules (e.g. AKT and ERK1/2) in both neonatal and adult CMs and stimulates cardiac myocyte cell cycle activity via signaling through the IL13R?1/IL4R? receptor heterodimer. Subsequent in vivo studies show that IL13 genetic deletion decreases CM cell cycle activity and abundance of M2-like immune cells, and inhibits heart regeneration in neonates, while administration of recombinant IL13 is capable of promoting CM cell cycle activity and extending the neonatal regenerative window. Here, we propose to identify the cellular mechanisms by which IL13 promotes heart regeneration post MI in neonatal mice and test the extent in which these mechanisms exist in the adult. Aim 1 tests the hypothesis that IL13 signals directly on CMs via the IL13R?1/IL4R? receptor to promote cytokinesis of pre-existing CMs and heart regenerating in the neonate and adult. Aim 2 will investigate the cellular source of IL13 following MI at neonatal and adult stages and will test the hypothesis that IL13 promotes tissue resident macrophage polarization via IL13R?1/IL4R? signaling and these macrophages facilitate post MI healing in the neonatal, but not the adult heart. Collectively, the proposed experiments will determine the cell type specific mediators of IL13 signaling during neonatal regeneration and will assess the preservation, or absence, of these mechanisms in the adult. By comparing the IL13-depentent mechanisms in CMs and the immune system that differentiate the regenerative (neonatal) and non-regenerative (adult) mouse heart, we aim to identify actionable pathways for augmenting regenerative capacity in the adult heart.